Abstract: This paper deals with the additional "structural strength" caused by the secondary consolidation of clay in subsoils.Two problems are covered in this paper. On the first one, the mechanism of yielding pressure PCQ for quasi- overconsolidated clay (QOC clay) is discussed.Then, the "Overshoot" of compression curves for aged clay was tested. It is verified that such a phenomenon is an essential behaviour for QOC clay. On the second one, the comparison of stress-strain-strength characteristics between QOC clay and OC clay was performed, and it is shown that an essential distinction between the quasi-over-consolidation characteristics due to aging and the overconsolidation characteristics due to unloading is presented.